Add @Required annotation

[ghost]

Adds a new @Required annotation to the model package for use in marking attributes in model classes as "required". This is so we can annotate attributes of our model classes that are expected to be present on every instance of the object, so developers can safely access without any explicit null checks. Meant to be a drop-in replacement for in-line code comments or javadocs that state the same (but apply no actual enforcement at runtime)

Category (place an x in each of the [ ])

• bolt (Bolt for Java)
• bolt-{sub modules} (Bolt for Java - optional modules)
• slack-api-client (Slack API Clients)
• slack-api-model (Slack API Data Models)
• slack-api-*-kotlin-extension (Kotlin Extensions for Slack API Clients)
• slack-app-backend (The primitive layer of Bolt for Java)

Requirements

Please read the Contributing guidelines and Code of Conduct before creating this issue or pull request. By submitting, you agree to those rules.

[salesforce-cla]

Thanks for the contribution! Before we can merge this, we need @fst-john to sign the Salesforce Inc. Contributor License Agreement.

[codecov]

Codecov Report

❌ Patch coverage is 35.71429% with 18 lines in your changes missing coverage. Please review.
✅ Project coverage is 73.28%. Comparing base (797d1c1) to head (61394ab).

Files with missing lines	Patch %	Lines
...om/slack/api/util/json/RequiredAdapterFactory.java	35.71%	14 Missing and 4 partials ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #1556      +/-   ##
============================================
- Coverage     73.28%   73.28%   -0.01%     
- Complexity     4513     4520       +7     
============================================
  Files           477      478       +1     
  Lines         14274    14302      +28     
  Branches       1487     1492       +5     
============================================
+ Hits          10461    10481      +20     
- Misses         2922     2925       +3     
- Partials        891      896       +5     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[ghost]

Apparently to get the "CLA is signed" check to pass we need to close this PR and open a new one; so doing that!